Add support for flattening in GeometryMapper.

third_party/WebKit/Source/platform/graphics/paint/GeometryMapper.cpp

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "platform/graphics/paint/GeometryMapper.h"
 
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/geometry/LayoutRect.h"
 
 namespace blink {
@@ skipping 355 matching lines @@
     success = false;
     return m_identity;
   }
 
   // Iterate down from the top intermediate node found in the previous loop,
   // computing and memoizing transforms as we go.
   for (auto it = intermediateNodes.rbegin(); it != intermediateNodes.rend();
        it++) {
     TransformationMatrix localTransformMatrix = (*it)->matrix();
     localTransformMatrix.applyTransformOrigin((*it)->origin());
+
+    // Flattening Lemma: flatten(A * flatten(B)) = flatten(flatten(A) * B).
+    // goo.gl/DNKyOc. Thus we can flatten transformMatrix rather than
+    // localTransformMatrix, because GeometryMapper only supports transforms
+    // into a flattened destination space.
+    if ((*it)->flattensInheritedTransform())
+      transformMatrix.flattenTo2d();
+
     transformMatrix = transformMatrix * localTransformMatrix;
     (*it)->getTransformCache().setCachedTransform(ancestorTransformNode,
                                                   transformMatrix);
   }
   success = true;
   const TransformationMatrix* cachedMatrix =
       localTransformNode->getTransformCache().getCachedTransform(
           ancestorTransformNode);
   DCHECK(cachedMatrix);
   return *cachedMatrix;
@@ skipping 55 matching lines @@
     const TransformPaintPropertyNode*,
     const TransformPaintPropertyNode*);
 template const ClipPaintPropertyNode* GeometryMapper::lowestCommonAncestor(
     const ClipPaintPropertyNode*,
     const ClipPaintPropertyNode*);
 template const ScrollPaintPropertyNode* GeometryMapper::lowestCommonAncestor(
     const ScrollPaintPropertyNode*,
     const ScrollPaintPropertyNode*);
 
 }  // namespace blink